In the field of communications, the need for high-speed transmission of data, including video and audio, has continued to increase. Moreover, there has been an increase in the selection of services by which users can connect to a network, such as the Internet. Specifically, Internet Service Providers (ISPs) may allow for connectivity to the Internet through lower-speed connections at different rates, such as 56 kilobits/second, by employing a Plain Old Telephone Service (POTS) line. Other choices for connection, which are at higher speeds, into a network can include Integrated Services Digital Network (ISDN), Digital Subscriber Line (DSL) service, and cable modem service over a Radio Frequency (RF) cable line. Further, other types of content providers may enable a subscriber to receive different types of media, such as a video stream, audio stream, etc.
In a typical DSL network, a network element supports a wide variety of features to facilitate the management, allocation and distribution of IP addresses. Normally, the subscriber profile can be configured locally on the network element or can be retrieved from a remote server (e.g., a RADIUS (remote access dial in user server)). A subscriber profile determines how an IP address (and optionally the associated route for the subscriber LAN) would be provided to a certain subscriber. Similarly, a cable modem user uses a DHCP server to allocate IP addresses for the clients.
Traditionally, the IP addresses provided to the subscribers must exist within the IP address/subnet “boundary” of an interface. That is, an interface must “contain” or “subtend” the IP addresses that are being provided to subscribers. The network element only has to advertise (e.g., perform network broadcast) for the interfaces (not each IP address assigned to each interface). However, since those IP addresses are preprovisioned in relatively large blocks, it is not uncommon for them to be unused IP addresses (an IP address assigned a subnet that is not assigned to a subscriber due to a lack of demand cannot be used by another router). This is unfortunate because IP addresses can be a scarce resource.
When the IP addresses are not adequate, a default interface, also referred to as interface of last resort, may be used to provide additional IP addresses. In the default interface configuration, the IP addresses provided to the subscribers do not have to exist within IP address/subnet boundary of an interface. That is, the IP addresses provided to the subscribers are decoupled from the interface(s) definition. This allows a single IP subnet to be fully allocated and to be shared across multiple routers. However, the network element has to perform network broadcast for each IP address handed out as an interface of last resort. As a result, large amount of network traffic is generated. In addition, the IP address provider (e.g., DHCP or RADIUS) has to handle each individual IP address request as a last resort, which leads to a heavy load on the IP address provider.
Typically, there are two major approaches to provide IP addresses to the subscribers, traditional approach and a default interface (interface of last resort) approach.
Traditional Approach
This approach is more appropriate for a centralized aggregation model (e.g., BRAS). Utilizing the traditional approach, the following methods are supported on a network element to manage/allocate IP addresses to the subscribers:                1) Static Bridge-1483 encapsulated subscribers: Static IP addresses are allocated to these subscribers. Subscriber-specific static routes (usually for the subscriber LAN) can also be assigned.        2) DHCP Bridge-1483 subscribers: IP addresses are allocated to these subscribers utilizing an external or on-board DHCP server.        3) PPP-encapsulated subscribers: A statically or dynamically allocated IP address can be assigned per PPP session. A subscriber-specific static route (usually for the subscriber LAN) can also be assigned at session bring-up time.        4) Route-1483 encapsulated subscribers: Static IP addresses are allocated to these subscribers. A subscriber-specific static route (usually for the subscriber LAN) can also be assigned.        
In all cases above, an IP interface is configured on the network element that “contains” the IP addresses that are being assigned to the subscribers. For example, following interface may be defined on the network element, or if multiple virtual routers are supported, in a given context (a virtual router or a physical router):                Interface Subscribers        IP address 10.10.10.1 255.255.255.0The IP addresses assigned to the subscribers (whether static, DHCP or PPP) fall within a range of 10.10.10.2-10.10.10.254. In summary, the IP addresses provided to the subscribers are dependent on the interface(s) configured on the network element.Default Interfaces or Interfaces of Last Resort        
This approach is more commonly used in a more distributed aggregation model. It has also been traditionally used for PPP encapsulated subscribers in a Remote Access dial-up environment. The default interface feature of the network element would provide the capability to decouple IP address assigned to a session from the interface IP address/subnet mask definition. The application of this feature to PPP-encapsulated subscribers would be equivalent to the PPP-default interface feature on the network element. Default Interfaces are applicable to both PPP-encapsulated and CLIPs-encapsulated subscribers.
The default interface for PPP subscribers may be configured to provide PPP sessions an interface to which they can bind in case no other valid interface exists (e.g., a valid interface is one whose IP subnet “contains” the IP address of the subscriber) on a system. Hence also the name: “interface of last resort”. Normally, a PPP session that cannot bind to an interface (due to lack of an interface with a valid matching IP range) simply fails the binding. With the use of default interfaces, this PPP session will instead bind to the interface designated as a “default” interface. The default interface in this instance acts as an interface of last resort. By using such a design, there is no requirement to have all subscribers terminated on a single router interface be assigned addresses from a common IP subnet. This allows for an IP subnet to be shared across many router devices. This allows the service provider to more fully utilize the IP address space allocated to them, as there are no wasted addresses due to allocation inefficiencies (however, IP addresses assigned to a subnet interface may still go unused depending on demand). It also allows the service provider to build redundancy into the access network, provided there is a means to reroute subscriber sessions to a standby router. The default interface works in a similar manner as described above except that the server involved here would be the DHCP server.